This invention concerns a high-alumina refractory cement and a process for producing the same.
One important purpose of the invention is to provide a cement with good refractory properties, but which can be used under conditions similar to those applicable to conventional cements. This new refractory cement also offers the general advantage of allowing simple starting materials to be used.
This invention relates to a high-alumina refractory cement which contains, in addition to ingredients adapted to form a hydraulic bond, one or more ingredients adapted to form a phosphatic chemical bond.
This new high-alumina refractory cement offers an unexpected solution to the problem of deficiencies in performance that can occur at intermediate temperatures with concretes obtained using only aluminous binders. Previously, this problem was solved by mixing the ingredient designed to produce a hydraulic bond with a filler; this produces ceramic bonds at intermediate temperatures, at which hydraulic bonds deteriorate. The ceramic bonds result from sintering of the filler with the other ingredients, at intermediate temperatures. This premature ceramization ensures the strength of the concrete when the hydraulic bond deteriorates at these intermediate temperatures.
The high alumina refractory cement according to the invention offers high performances at intermediate temperatures.
This invention in fact concerns a high-alumina refractory cement which, in addition to ingredients adapted to form a hydraulic bond, and conventional secondary ingredients, contains one or more ingredients adapted to form a phosphatic chemical bond. This phosphatic chemical bond offers an unexpected cheap, simple solution to the problem of the zone of weakness found at intermediate temperatures in concretes made from conventional high-alumina cements.
The secondary ingredients consist of compounds normally used in this field, such as monosodium citrate, and carboxylic acids and their salts.
In one embodiment of the invention, the ingredients designed to promote the phosphatic chemical bond consist of 0.5 to 5% by weight (expressed as phosphorus pentoxide P.sub.2 O.sub.5), in relation to the weight of high-alumina cement, of at least one soluble alkaline phosphate. The content of soluble alkaline phosphate, expressed as P.sub.2 O.sub.5, is preferably between 1.5 and 5%. Moreover, in another embodiment of the invention, the soluble alkaline phosphate is sodium hexametaphosphate, tripolyphosphate or pentapolyphosphate. It is preferably sodium hexametaphosphate.
In one embodiment of the invention, the ingredients adapted to form a hydraulic bond consist of a clinker obtained from aluminous materials on the one hand, and calcerious or dolomitic materials on the other.
According to another feature of the invention, the ingredients adapted to form a hydraulic bond essentially comprise a clinker containing at least 35% by weight of monocalcic aluminate (CA) in relation to the weight of clinker.
According to yet another feature of the invention, the aluminous material forms 65 to 95% of the weight of high-alumina refractory cement, and consists at least partly of calcined alumina.
According to one feature of the invention, the high-alumina refractory cement has a minimum specific surface area measured by gas flow (Blaine) of 4300 cm.sup.2 /g.
This invention also relates to a process for manufacturing this new high-alumina refractory cement, in which cement clinker and calcined alumina are ground together, and the phosphatic ingredient and secondary ingredients are mixed in after grinding.
The invention also relates to a process for manufacturing this new high-alumina refractory cement, in which a ground clinker, a calcined alumina which has previously been ground, the phosphatic ingredient or ingredients, and secondary ingredients, are mixed together.
The clinker is preferably ground to ensure a Blaine specific surface area of approximately 3000 to 5000 cm.sup.2 /g, and preferably approximately 3600 to 4000 cm .sup.2 /g.
According to one feature of the invention, the high-alumina refractory cement is obtained by mixing:
20 to 90% by weight of clinker containing at least 35% CA
2 to 79% by weight of calcined alumina with a specific surface-area of between 0.3 and 50 m.sup.2 /g, measured by gas adsorption, (BET).
0.5 to 5% by weight of soluble alkaline phosphate, expressed in percentage weights of P.sub.2 O.sub.5.
The clinker used is obtained by the fusing or sintering at between 1400.degree. and 1700.degree. C. of aluminous materials and calcitic and/or dolomitic materials. Limestone, magnesian limestone, dolomitic limestone, each possibly decarbonated, and similar substances can be used for this purpose.
The aluminous material used to make the clinker is selected from bauxites, laterites or other aluminous ores and particularly red bauxites, white bauxites with low iron oxide content, or hydrated or calcined aluminas. The BET specific surface-area of the calcined alumina added to the clinker is preferably between 0.5 and 10 m.sup.2 /g.